Conventional dual slope integrator analog to digital converters are generally preferred where low voltage input signals are to be measured. The dual slope technique provides excellent noise rejection and an accuracy that in general is independent of the clock pulse rate and integrating resistors and capacitors. However, such known circuits have a first order of sensitivity to zero drift and second order of sensitivity to span error with temperature. Further, it is generally difficult to obtain bipolar operation using conventional known techniques. In its practical form, the accuracy of an analog to digital convertor depends upon the following characteristics of the basic circuit:
1. Offset voltage of the pre-amplifier or buffer, integrator and crossover detector.
2. Offset voltage drift of the pre-amplifier or buffer, integrator and crossover detector.
3. Dynamics of the switches, capacitors, pre-amplifier, integrator and crossover detector.
4. Discontinuities at or near zero signal associated with achieving bipolar signal conversion.
5. Internal circuit noise.
6. Linearity of the amplifier, integrator and crossover detector.
Any analog to digital conversion circuit which would provide for improvement in the foregoing characteristics would be highly desirable.
In my U.S. Pat. No. 4,107,618, there is described an operational differential amplifier system wherein the signal gain of an input analog signal can be made independent of a reference voltage. The technique described in this patent can be used to advantage in the provision of a greatly improved analog to digital convertor wherein zero drift is essentially eliminated.
In U.S. Pat. Nos. 4,608,553 and 4,390,864, there is described an analog to digital convertor without zero or span drift employing a technique using fixed slope, variable amplitude analog to digital conversion.